Linux compilation fix and README.linux updates.

Fix Linux compilation following the recent requirement that
wxWidgets be built with "-std=gnu++0x".  If you don't use the
top level makefile and instead use xLights.cbp.mak directly,
the wxWidgets compile change will also mean that you need
to change your xLights compile command as noted in the README.linux
file.

Update one more place in the README.linux file that mentioned
the older wxWidgets 2.9.4 requirement.  Also fix the git repository
URL in the file as well.